Project Summary/Abstract The primary goal of this project is to test three hypotheses regarding glaucomatous damage to the visual system. First, that clinically detectable neural, glial and connective tissue alterations occur deep in the optic nerve head (ONH) at a very early stage in the pathophysiology of glaucomatous damage to the visual system. Second, that the location and magnitude of the earliest of these ONH changes, detectable in vivo by spectral domain optical coherence tomography (SDOCT), predict the specific locations of subsequent alterations of the peripapillary retinal nerve fiber layer (RNFL) and orbital optic nerve axon loss. Third, that ONH connective tissue structural stiffness is altered both by age and glaucomatous damage and that it underlies the clinical appearance of the glaucomatous optic disc, specifically by influencing the depth of glaucomatous ONH structural change or cupping. Until now, all animal models of glaucoma have been studied in isolation from human glaucoma. A second goal is to demonstrate that our hypotheses and techniques have evolved to a point where both can be simultaneously tested in monkeys (Specific Aim 1) and humans (Specific Aim 2). Aim 1 is to characterize the onset and progression of SDOCT ONH structural change within pre- and post- laser SDOCT ONH data sets from both eyes of 70 unilateral experimental glaucoma (EG) monkeys. Aim 2 is to characterize the onset and progression of ONH structural change within longitudinal SDOCT ONH data sets from 250 human ocular hypertensive and early glaucoma patients. The methodology includes: longitudinal Heidelberg Spectralis 870 and 1060 nm SDOCT ONH image acquisition in monkeys (870 nm only in humans); their visualization, delineation and quantification within custom Multiview software; and in monkeys only, post- mortem 3D histomorphometric ONH reconstruction and quantification, co-localized, eye-specific comparison of SDOCT ONH and 3D histomorphometric reconstructions and regionally-aligned orbital optic nerve axon damage map generation using our custom Axonmaster axon counting software. The expected outcomes are: 1) deep ONH structural change will occur before and predict subsequent ONH surface and RNFL change during the onset and progression of glaucomatous damage in both monkey and human eyes; 2) early ONH structural change will co-localize to orbital optic nerve axon loss in monkey eyes and precede RNFL alterations in both monkey and human hypertensive eyes; 3) in monkeys, younger ONHs will be more compliant and older ONHs will be stiffer when normal, and both will demonstrate transient hypercompliance followed by progressive stiffening as glaucomatous damage progresses; 4) younger monkey and human ONHs will demonstrate a deeper form of cupping than older ONHs; 5) in younger compared to older eyes, the onset and progression of structural change (cupping) will include a larger connective tissue component; and 6) 1060 nm SDOCT imaging will improve visualization of deep monkey ONH imaging targets compared to the existing clinical standard (870 nm imaging).